"According to the Defense Department's annual "Base Structure Report" for fiscal year 2003, which itemizes foreign and domestic U.S. military real estate, the Pentagon currently owns or rents 702 overseas bases in about 130 countries and has another 6,000 bases in the United States and its territories."

Which sort of puts it in perspective. As to the why, I would imagine that most are hang-overs from previous conflicts (WW II; Korea) and the "Cold War".